The invention relates to a glass halogen filament tube comprising a multi-layer optical interference coating that functions to increase the efficiency of the filament tube by at least 26% as measured by LPW gain. It will be appreciated by those skilled in the art to which this invention pertains that selected aspects may find use in related applications where improvement of LPW and lamp life are of concern.
Halogen filament tubes are known. Generally, the premise of the halogen IR filament tube, sometimes referred to herein as a halogen IR lamp, is to provide a filament tube having a spectrally reflecting filter on the outside of the tube that functions to reflect a portion of the emitted IR radiation back to the filament, where a fraction of that reflected radiation is absorbed. The absorbed radiation improves filament tube performance by reducing the input electrical power needed to operate the filament tube at a constant filament temperature, i.e., it increases efficacy of the filament tube. The filter also is designed to optimally transmit as much of the visible radiation as possible in order to maintain not only lumen output, but also the color of the light generated by the filament tube.
Conventional halogen IR filament tubes are constructed using quartz as the envelope material. Quartz has been the material of choice for halogen IR filament tubes for several reasons, including its structural robustness, its stability at high temperatures of operation, and its compatibility with high temperature CVD (Chemical Vapor Deposition) coating processes. Filters or coatings are used to modify and/or enhance the performance characteristics of filament tubes. For example, U.S. Pat. No. 5,138,219 to our common assignee and incorporated herein by reference in its entirety, discloses an optical interference coating comprising alternating high and low refractive index material layers for transmitting visible radiation and reflecting IR radiation. Exemplary coatings are deposited on the outer surface of an envelope comprising a vitreous light-transmissive material, such as quartz, capable of withstanding high temperatures of about 800° C. The composition of the filter employed is critical to the goal of enhancing lamp performance. There are many different coating designs available today, and on many different types of lamps, to achieve not only very specific performance parameters for specialty lighting needs, but also to enhance the performance and life of lamps for more general every day uses. Even with the advantages gained using such coatings, quartz filament tubes continue to suffer from drawbacks due to expense, i.e., they can be costly to manufacture due not only to the cost of the materials, but also to the cost of processing at the high temperatures necessary when using quartz.
A more attractive filament tube material may be glass, given that glass is less expensive than quartz and is processed at lower temperatures. Even though glass potentially offers a more cost effective alternative, it is not widely used due to the susceptibility of the glass to experience structural failure when exposed to excessive film stress. Further, known glass lamps, even when coated to enhance performance, have not exhibited lumens per watt (LPW) gains of more than about 20%, and usually only achieve lesser gain.
What has not been known, therefore, is a glass halogen tube having deposited on the exterior surface thereof a high performance optical interference coating that transmits radiation in the visible portion of the spectrum, from about 400-750 nm, and reflects radiation in the infra-red portion of the spectrum, from about 800-2500 nm, wherein the glass filament tube operates at a temperature of about 600° C., without filament tube structural failure issues of the glass substrate related to excessive film stress caused by the interaction of the coating materials and the glass at operating conditions.
It would be desirable to provide a glass halogen, optical interference coated filament tube that with specified filament tube parameters, achieves measured LPW gains in excess of 26%, and even more than 30% LPW gain. In fact, the filament tubes of this invention, as compared to uncoated filament tubes of similar composition and structure, prepared to meet the specified ratings, have shown measured LPW gains of ˜36%, which is almost a two-fold improvement in performance.